For over 30 years complexes with the general formula [NiPh(P,O)L] (L = tertiary phosphine; P,O = chelating phosphanylenolato ligand) have been used as highly efficient oligomerisation catalysts suitable for the production of linear α-olefins. The same complexes, which are usually referred to as SHOP-type catalysts (SHOP = Shell Higher Olefin Process) can also be used as ethylene polymerisation catalysts, provided they are treated with a phosphine scavenger that selectively removes the tertiary phosphine ligand (L). This Perspective examines the impact of various parameters (influence of the substituents, backbone size, solvent, use of cocatalysts, etc.) on the catalytic outcome of the complexes. Overall, this review shows that the selectivity and activity of the catalyst may be tuned efficiently through directed modification of the P,O chelator.

A series of calix[4]arenes bearing two phosphorus pendent groups p-Bu^t-calix[4]arene-25,26-[CH₂P(O)Ph₂]₂-27-(OR¹)-28-(OR²) (R^{1 or 2} = CH₂CO₂Et, R^{2 or 1} = H, 2; R¹ = R² = CH₂CO₂Et, 3; R^{1 or 2} = C(O)Ocholesteryl, R^{2 or 1} = H, 4; R¹ = R² = C(O)Ocholesteryl, 5; R^{1 or 2} = (R)-CH₂C(O)NHCMe(Ph)H, R^{2 or 1} = H, 6) and p-Bu^t-calix[4]arene-25,26-(CH₂PPh₂)₂-27-(OR¹)-28-(OR²) (R¹ = (R)-CH₂C(O)NHCMe(Ph) H, R² = H, 7; R¹ = (R)-CH₂C(O)NHCMe(Ph)H, R² = SiMe₃, 8; R¹ = (R)-CH₂C(O)NHCMe(Ph)H, R² = (R)-CH₂C(O)NHCMe(Ph)H, 10) have been synthesised. The enantiomerically pure calixarenes 7, 8 and 10 having an AABC substitution pattern are inherently chiral. Reaction of the latter three diphosphines with [Pd(2-Me-allyl)(THF)₂]BF₄ (THF = tetrahydrofuran) afforded the chelate complexes [Pd(2-Me-allyl)(diphosphine)]BF₄ 11–13, respectively, while reaction with [Rh(NBD)(THF)₂]BF₄ (NBD = norbornadiene) resulted in quantitative formation of the complexes [Rh(NBD)(diphosphine)]BF₄ 14–16, respectively. As a result of allyl rotation, the palladium complexes 11–13 exist in solution as two interconverting species. These complexes efficiently catalyse the alkylation of 1,3-diphenylprop-2-enyl acetate with dimethyl malonate, the turnovers being ca. 30 h⁻¹. Enantioselectivity was shown to depend on the size difference between the B and C substituents. Thus, while virtually no induction was observed with the chiral calixarene 10 bearing two identical substituents, ee’s of 45% and 67% respectively were observed with 12 and 11, which have a more marked dissymmetry. Similar trends were observed in the catalytic hydrogenation of dimethyl itaconate with the rhodium complexes 14–16, leading to ee’s of 48%, 25% and 0%, respectively. Related chiral calixarenes in which the two phosphine arms occupy proximal instead of distal phenolic positions were found to be considerably less effective in catalysis of both allylic alkylation and hydrogenation.

The potential of molecules that combine the properties of a conical cavity with those of a covalently-linked transition-metal centre is highlighted through the assessment of cyclodextrin- and calixarene-derived podands (“cavitand” ligands) in coordination chemistry and catalysis. Metallocavitands with coordination sites directed towards the interior of the generic cavity provide interesting systems for studying host–guest complexation processes, their enhanced strength of metal-ion binding allowing for regioselective catalysis in a confined environment, and stabilisation of coordination complexes of unusual forms. Where cavitands have exo-oriented podand arms, the intrinsic dynamics of the cavity can dramatically modify metal chelation behaviour and the catalytic properties of the complexes. Such functionalised cavities are also useful as metal-ion transporters.